1. If I purchase a lovely and fast Mac running Maverick, is there any reason why I should not downgrade the whole OS to Snow Leopard?
Currently produced Macs cannot natively boot nor run Snow Leopard. The last Macs to do so were produced in 2011. Hence the need to run Snow Leopard in Parallels to restore Rosetta and PPC app capability.
2. If I firewire-connect my older G4 PPC iMacs running Tiger,
a) will I be able to access the files as if they were external drives?
Not sure about "networking" using Firewire, but if connected via Ethernet cable (or wireless) and Networking turned on, you could.
b) would I be able to run and use my older QuarkXPress and CS2 apps?
Not sure what you are asking here, but you could run Quark and CS2 on your older G4 and use Screen Sharing over ethernet or wireless to see and control the G4 from a newer Mac.
3. Or, will my older iMacs only be available for use (connected via Firewire) in Target mode... or, not even then?
In Target mode, your G4's hard drive will appear as an external drive connected to the new Mac and then would be accessible by a newer Mac. It will probably be best to make new copies of your G4 files on your new Mac; perhaps using Migration Assistant.
Many Thanks,
Precisely